The principle of measuring based on a capacitive pressure sensor has proved to be a simple and reliable way of measuring pressure. Capacitance measuring is based on a change in the gap between two surfaces of a pair of electrodes in the sensor. The capacitance between the surfaces, i.e. the storage capacity of electric charge, is a function of the area of the surfaces and the distance between the surfaces. Capacitance measuring can be used even at fairly low ranges of pressure.
The basis for the structure of small capacitive pressure sensors is a thin-film structure manufactured on silicon. The thin membrane forming one of the electrodes bends under influence of pressure and thus changes the capacitance to be measured.
Anisotropic wet etching of silicon is an inexpensive and relatively precise way of manufacturing thin-film structures in silicon. Its biggest drawback is the area occupied by structures defined by the extremely slowly etching sloping crystal planes {111}.
In anisotropic etching, the speed of the etching depends on the orientation of the crystal. Extremely slowly etching crystal planes are so called etching inhibitor planes, and they define the generated structure. On the other hand, the geometry of the structures is limited by the crystal structure and there are a limited number of basic structures. An aqueous solution of potassium hydroxide, KOH, for example, can be used as an anisotropic etching agent for silicon.
In silicon, the etching inhibitor planes are the crystal planes {111]. Silicon material is etched away by the etching agent, until it meets a crystal plane {111]. Should it be desired to end the etching at these planes, the structures have to be planned such, that they do not contain jutting corners, since the corner is going to be etched underneath. The etching will be stopped only at inward corners.
Prior art will be described below with exemplifying reference to the attached drawings, in which:
FIG. 1 shows the progress, as a function of time, of wet etching according to prior art, using two different etching masks,
FIG. 2 shows the membrane structure of an ideal pressure sensor according to prior art,
FIG. 3 shows the membrane structure of a pressure sensor obtained by anisotropic wet etching according to prior art, and
FIG. 4 shows a silicon wafer of type (100) according to prior art, in which wafer membrane structures of a pressure sensor have been manufactured by anisotropic wet etching.
FIG. 1 shows the progress, as a function of time, of wet etching according to prior art, using two different etching masks. The figure shows, as initial conditions, a structure made on a silicon wafer of type (100) using a square etching mask 1, as well as a structure made on a silicon wafer of type (100) using a diamond shaped etching mask 2. In the figure, time progresses from left to right. Using a square etching mask, oriented in the direction <110>, the etching progresses evenly with no intruding etching 3, 4. Correspondingly, when using diamond shaped etching masks, oriented in the direction <010>, etching proceeds underneath the mask 5, 6.
In FIG. 1, etching is stopped before the shapes corrode into inverted pyramids. Identical structures 7, 8 are obtained as the end result of the etching. At the bottom of the figure, there is a sectional view of the generated structures. The final shape of the pattern is defined by the crystal planes {111}. The planes {111} form an angle arctan(1/√{square root over (2)})=54.74° with the crystal plane (100) 9. The figure clearly shows the way in which the sloping planes decrease the area remaining in the center.
FIG. 2 shows an ideal pressure sensor membrane structure according to prior art. However, the ideal structure 10 depicted in the figure cannot be produced by wet etching into silicon (100).
FIG. 3 shows a pressure sensor membrane structure obtained by anisotropic wet etching according to prior art. In the structure 11 obtained through anisotropic wet etching depicted in the figure, the sloping surfaces consume √{square root over (2)}d of the length of the side, where d is the depth of the etching.
The most severe drawback of anisotropic wet etching is the surface area occupied by structures defined by the sloping extremely slowly etching crystal planes {111}. Often the problem of surface area loss is solved by using dry etching DRIE (Deep Reactive Ion Etching), but the evenness of the depth of the etching is not nearly so well controlled as in wet etching. Dry etching can further be performed just one wafer at a time, whereas wet etching can be performed on a batch of wafers at a time. In wet etching, the membrane structure of a pressure sensor is often given its finish by patterning the surface area of the flexible membrane portion on the back surface of the silicon structure.
FIG. 4 shows a silicon wafer of type (100) according to prior art, in which wafer pressure sensor membrane structures have been produced by anisotropic wet etching. Sixteen square etched surface areas 13 are depicted on the silicon wafer 12 according to prior art. In the figure, the area 14 of the flexible membrane portion, which is patterned on the back side of the membrane, is indicated by a circle. The cutting lines of the pressure sensors are drawn in heavy black lines.
In the structure of a pressure sensor according to prior art, a problem is the surface area occupied by the structures of the, in anisotropic wet etching of silicon, extremely slowly etching sloping crystal planes {111}. The sloping planes also diminish the area of the membrane portion remaining in the center of the pressure sensor.